Mechanical stress can be detrimental for the operation of integrated circuits and other semiconductor components. Packaging induced stress is known to affect significantly the performance of transistors and circuits. Similarly, 3D interconnects such as through-substrate vias (TSVs) are known to introduce stress in the active regions of the wafer. It is therefore necessary to evaluate the stress in terms of the stress components of the stress tensor which consists of three normal components σxx, σyy and σzz and three shear components σxz, σyz and σxy, defined with respect to orthogonal directions x, y and z with the xy plane corresponding to the plane of the semiconductor wafer from which the IC or other component is fabricated.
One approach to detecting package-induced stress is based on the piezo-resistive effect in diffused resistors or transistors. One example of a piezo-resistive stress sensor built on this principle is the rosette sensor described in detail in document “Silicon piezo-resistive stress sensors and their application in electronic packaging,” Suhling and Jaeger, IEEE Sensors Journal, vol. 1, no. 1, pp. 14-30, June 2001. The shift in resistance of complementary type diffused resistors oriented along different directions is monitored to deduce the stress present in the active material. To compensate for the impact of temperature variations during measurements, these resistance shifts are subtracted one from the other so as to cancel the impact of thermal effects on resistors. These types of sensors however suffer from three main limitations. Firstly, they are not sensitive to out-of-plane shear stress components on a wafer surface, which is the one commonly employed for CMOS technology. Secondly, the resistor sensitivity to σzz does not change with a planar rotation of the sensor on a surface, so its contribution is regularly cancelled out with the one from the temperature. Thirdly, it is only possible to extract the difference (σxx−σyy) and not the two components σxx and σyy individually, once again due to the need to compensate for temperature.
To determine the remaining coefficients (σxz, σyz, σzz, and (σxx+σyy)), one approach is to build rosette sensors on a surface, where the transformed piezoresistive matrix provides more orientation-dependent coefficients. Therefore, variations of resistances oriented in different directions due to stress have the potential to provide an independent family of linear equations from which the stress components can be extracted. Nonetheless, the in-plane resistivity matrix being symmetric, it features only 3 independent components. Any in-plane rotation of the resistor can only result in a combination of these three coefficients. Thus at most 3 independent equations can be extracted from a single-polarity rosette sensor, and one is inevitably lost for temperature compensation. In any case, all the sensors based on a surface cannot be processed in conventional CMOS technology where the wafer surface is oriented along the direction.
Another solution reported in literature is to create a non-planar current on a oriented wafer, as illustrated by documents: “Towards piezo-resistive CMOS sensors for out-of-plane stress”, Lemke et al, Proceedings, IEEE 22nd International Conference on Micro Electro Mechanical Systems, March 2009 and “Piezoresistive CMOS sensors for out-of-plane shear stress”, Baumann et al, Proceedings IEEE Sensors Conference, 2009. In these sensor designs, the current is forced below a shallow trench isolation (similarly to an STI diode), which makes it non-planar. This principle can be used to detect both σzz and the out-of-plane shear stress σxz and σyz. However the extraction of the σzz, σxz, and σyz is very challenging. Indeed, a large part of the current trajectory (below the spacer) is non-vertical. As a result, the structure must be repeated with different STI widths to de-embed σzz from all the others as explained in the above-identified reference by Lemke et al. In the pseudo-Hall sensor used in the above-identified reference by Baumann et al, no solution is proposed to de-embed between the contribution of σxz (vertical current on the sidewalls of the STI) and σxy (horizontal current under the STI).
V-groove stress sensors are a well-known technique used in the MEMS industry to fabricate sensors. In particular, it has also been used to build membrane sensors that can detect a pressure applied by an external force on the chip. However in that case the V-groove is used simply to build a mechanical structure. The electrical characteristics of the resulting slanted surfaces are not exploited. For example in document U.S. Pat. No. 6,150,681, the piezo-resistive sensors are put on the membrane and not on the slanted surfaces themselves. Furthermore, all piezo-resistive sensors relying on a free membrane or an internal cavity are not suitable for detecting packaging-induced stress precisely because they are sensitive to the external stress (which they are meant to detect).